A wireless communication transmitter conditions a signal for transmission over a channel, such as by converting that signal to one having an amplitude and frequency more suitable for transmission. The resulting transmit signal, however, may have a phase which is shifted relative to the signal input to the transmitter (e.g., due to a biasing change of, and/or a gain change of, and/or the non-linearity of, an amplifier or other circuit included in the transmit path). A transmitter may compensate for this undesired phase shift by applying the opposite rotation to the phase of the input signal.
Of course, the magnitude and direction of the phase shift fluctuates with changes in the characteristics of certain transmit path circuits. Changes in the gain of an amplifier, for example, may alter the magnitude and direction of the phase shift introduced by that amplifier. In this case, determining the phase shift introduced at any given time is particularly complicated in Code Division Multiple Access (CDMA) or Long Term Evolution (LTE) systems where such gain changes occur dynamically to maintain control over the transmit signal power.
Prior approaches to determining transmitter phase shift include the use of special test signals, which is undesirable in terms of live operation. Other approaches require undesirably complex trigonometric calculations or look-up tables, for determining which phase shift corresponds to a given operating condition.